Not sure if this should go here or in the relativity forum... let me know if I need to move it. I was reading something, and it said this: "A photon also has a rest mass, which is zero, even though a photon is always moving at the speed of light, and so is never at rest. But if an observer moves fast enough in the same direction as a photon, sees the photon as having less and less energy. By chasing the photon, it can be made to have as little mass-energy as you like. As you chase it faster and faster, the photon looks redder and redder, by doppler shift, and the energy of a very long-wavelength photon approaches zero as you approach the speed of light." Now, in my limited knowledge of relativity, light is always supposed to go the same speed, no matter what speed you are traveling, right? Are they trying to say something different here, or are they just saying that because the energy is related to the wavelength, the dopler effect can basically make the wavelength go to infinity and cause the energy to go to 0? If that is true, doesn't that present and interesting problem of the energy of light/photons being completely relative, even if they are still moving (a moving photon particle still moving at the speed of light would have effectively 0 energy, which doesn't make sense to me)? I could be completely wrong here, but it was just something I was wondering. My other question is a little quicker. As something goes faster and faster, therefore gaining mass according to relativity, does the object become more dense? Bigger? etc... I have a thought about that, but if there is a concrete answer for it already, I would like to hear that. Thanks for clarifying.